An amorphous oxide film composed of indium oxide and zinc oxide, or indium oxide, zinc oxide and gallium oxide has attracted attention as a transparent conductive film or a semiconductor film (used in a thin film transistor or the like) since it has transmittance to visible rays and a wide range of electric properties from a conductor or a semiconductor to an insulator.
In particular, since an n-type semiconductor material containing indium oxide and zinc oxide has been found by Hosokawa et al., (Patent Document 1), various oxide semiconductors containing indium oxide and zinc oxide have attracted attention.
As the method for forming the above-mentioned oxide film, a physical film-forming method such as sputtering, pulse laser deposition (PLD) and deposition or a chemical film forming method such as a sol-gel method have been studied. Of these methods, a physical film-forming method such as sputtering has been mainly studied since a film can be uniformly formed in a large area at a relatively low temperature.
When forming an oxide thin film by the above-mentioned physical film-forming method, it is common to use a target formed of an oxide sintered body in order to form a uniform film stably and efficiently (at a high film-forming speed).
As the target for forming the above-mentioned oxide film (mainly, a sputtering target), studies have been mainly made on one having a composition of a known crystal form such as In2O3(ZnO )m (m=2 to 20), InGaZnO4 and In2Ga2ZnO7 or one having a composition close to that of these crystal forms.
Specifically, a target which is formed of a sintered body of an oxide which comprises mainly In and Zn and contains a hexagonal compound represented by the general formula In2O3(ZnO)m (m=2 to 20) or a target obtained by doping this oxide with at least one kind of an element having a valency of positive trivalency or higher in an amount of 20 at.% or less is disclosed (Patent Document 2).
Further, a target having a crystal structure of a hexagonal compound such as InGaZnO4 and In2Ga2ZnO7 (homologous crystal structure) has been studied (Patent Documents 3, 4 and 5).
Further, studies have been made on development of a target utilizing the properties of a mixture. For example, development of a target formed of a mixture of a hexagonal compound represented by In2O3(ZnO)m (m=2 to 20) and In2O3 or a target formed of a mixture of a hexagonal compound represented by In2O3(ZnO)m (m=2 to 20) and ZnO (Patent Document 2), a target formed of a mixture of a hexagonal compound represented by InGaZnO4 and a spinel compound represented by ZnGa2O4 (Patent Document 6) or the like has been studied.
In addition, Patent Document 7 discloses an oxide represented by InGaO3(ZnO)m (m=1 to 20) such as InGaO3(ZnO)2 and a synthesis method thereof.
As for targets, no studies have been made on other oxides than those having the above-mentioned known crystal form, and thin films have been only studied (Patent Documents 3 and 8). Specifically, a thin film formed by a method in which the composition ratio is adjusted during the film formation by co-sputtering or the like has been studied.
Meanwhile, as for oxides which do not have known crystal forms, a change in solubility limit or lattice constant of a sintered body obtained by firing powder raw materials has been reported (Non-Patent Documents 1 and 2). Non-Patent Document 2 gives an example in which calculation is made on the assumption that an oxide having a crystal form represented by In1.5Ga0.5O3(ZnO)m is present. However, no specific studies were made on the synthesis of this oxide, application thereof to a sputtering target or the like (Non-Patent Document 2, TABLE IV).
On the other hand, studies have been made on producing various thin film transistors by changing the composition of indium oxide, zinc oxide and gallium oxide (Patent Document 9). However, since studies on targets at each composition were not sufficient, a thin film transistor obtained had a high specific resistance.
In addition, an example was disclosed in which an amorphous oxide semiconductor film and a thin film transistor were formed by using an In—Ga—Zn—O sintered body with a metal composition ratio In:Ga:Zn=30:15:55 (Patent Document 10).
However, there was a problem that the Ga content of a thin film was significantly decreased such that it became about two-third of the Ga content of the target. This suggests improper properties of the target, however, no studies were made on the target properties and the improvement thereof.